There are many ways to enhance your breasts and gain weight. You can take medication, apply cream & lotion and do breast enhancement exercises. For details of each, click here
How Do I Gain Weight In My Breasts?
Unfortunately there is no way for us to ensure the weight we put on our bodies goes straight to our breasts!
It all depends on our genetics, where the fat decides to go first...Some women are pre-disposed to having fat go to their breasts mostly before any other undesirable locations.
The only sure way to gain weight there and nowhere else is to get implants.
If you breastfed, they're probably gone forever, (unless you can afford implants). I wouldn't waste money on creams and herbs etc, I just got a good padded bra and pretend they're great!